THE CENTRAL NERVOUS SYSTEM. 347 
The ninth cranial nerve (ZX) (N. glossopharyngeus) arises 
from the area ovalis from a line craniad of the dorsal line of 
origin of the vagus roots (X) and between these and those of 
the auditory (V///). It arises by a number of rootlets which 
are larger than those of the vagus (X), with which this nerve 
is closely associated. 
The eighth cranial nerve (V//7) (N. acusticus) appears at 
the lateral end of the trapezium (/). It arises from an elevation 
(Fig. 141, 2) which is continued dorsomediad along the cranial 
border of the area ovalis. 
The seventh cranial nerve (V//) (N. facialis) leaves the 
lateral border of the trapezium (/) near its cranial edge, 
between the fifth and eighth nerves. It is much smaller than 
the eighth nerve. 
The sixth cranial nerve (V/) (N. abducens) arises by about 
six bundles from the groove between the pyramids and the 
trapezii and passes craniad. 
2. Metencephalon.—The metencephalon includes the pons 
and the cerebellum. 
The pons (Fig. 138, 2) is a mass of transverse fibres which 
forms the ventral and cranial part of the primitive hindbrain. 
It is a modification of the latter brought about by the develop- 
ment of the cerebellum, and the degree of its development is in 
direct ratio to that of the cerebellar and cerebral hemispheres. 
The pons forms a projecting mass of fibres which is marked by 
a median longitudinal groove, the sulcus basilaris (7), which 
indicates the course of the basilar artery (Fig. 121, ¢). Laterad 
the fibres of the pons converge somewhat and turning dorsad 
disappear in the cerebellum, forming the brachia pontis (Fig. 
141, @). ; 
The fifth cranial nerve (Fig. 138, /”) (N. trigeminus) arises 
by two roots from the caudal border of the pons, near the 
lateral end. The ventral root (4) is small; the dorsal one is 
much larger and soon forms the large semilunar ganglion (4) 
from which three branches (1,2, and 8) diverge. The ventral 
root (4) joins one of these branches (1). 
The cerebellum (Fig. 137, ///) is formed by an increase in 
size of the cranial portion of the primitive hindbrain. This 
